New generations of processing tools require tighter control of the gap between a wafer and a deposition source to meet composition and thickness uniformity across a wafer and between different wafers. Furthermore, processes may be conducted at various temperatures, and with a range of separations between the wafer surface and deposition source. Monitoring the uniformity of the separation distance for these processes can be important to ensure proper deposition thicknesses and quality.
Cameras have been used to ensure the uniformity of distance between the substrate support and processing fixtures, where the camera may be used to monitor a fixed section of the processing chamber and detect changes in the gap between the support and fixture or the presence of an object projecting above the level of the support when the object is in the camera's field of view.
Cameras, however, take up space either internally within the processing chamber or externally around the periphery of the chamber. Cameras are also limited to the temperatures they can be exposed to, so it may not be technically feasible to have cameras located within a chamber for higher temperature processes. For example, a camera and other optical detectors will overheat and cannot be used to track the movement of a wafer or its recess directly at process temperatures above about 80° C.
Cameras are also typically arranged around the outside edge of a wafer support, so cannot easily monitor an inside edge of the support.
Limited space in the chamber for viewing ports and optics limits options for remote imaging. High temperatures, plasma glow and interference from process-critical hardware limit options for remote optical measurement and optimization of wafer placement in the chamber.